Image-rejection I/Q demodulators

ABSTRACT

In a communications receiver for quadrature demodulation, a feedback technique for reducing the image response of the receiver. The communications receiver includes an I demodulator and a Q demodulator. A local oscillator (LO) signal is provided by a PLL to a quadrature LO generator that provides an LO_I signal to an I demodulator and an LO_Q signal to a Q demodulator. The LO_I and LO_Q signals are amplitude and phase-controlled versions of the LO signal. An image/signal ratio (I/S) detector detects the relative phase difference and the relative amplitude difference between the respective output terminals of the I demodulator and the Q demodulator and applies an amplitude control signal and a phase control signal to corresponding amplitude control and phase control inputs of the quadrature LO generator. The I/S detector calibrates the quadrature LO generator during the interstitial interval between the reception of data packets. The control signals from the I/S detector adjust the relative amplitude and phase of the LO_I and LO_Q signals in a manner that reduces the image response of the communications receiver.

INCORPORATION BY REFERENCE

This application, by this reference, hereby incorporates the followingU.S. patent applications, in their entirety, all filed on Jun. 12, 2000:

“Receiver Architecture Employing Low Intermediate Frequency And ComplexFiltering”, to Albert Liu, Ser. No. 09/592,016;

“Wireless Data Communications Using FIFO For Synchronization Memory”, toSherman Lee, Vivian Y. Chou, and John H. Lin, Ser. No. 09/593,583;

“Context Switch Architecture And System”, to Sherman Lee, Vivian Y.Chou, and John H. Lin, Ser. No. 09/592,009; and

“Dynamic Field Patchable Microarchitecture”, to Sherman Lee, Vivian Y.Chou, and John H. Lin, Ser. No. 09/672,064.

FIELD OF THE INVENTION

The invention relates to digital communications systems and, moreparticularly, to a technique for the reduction in the image responsecharacteristics of an integrated circuit receiver that incorporates I/Qdemodulation.

BACKGROUND OF THE INVENTION

I/Q (In-phase/Quadrature) modulators and demodulators are widely used indigital communications systems. I/Q demodulators are abundantlydiscussed in the technical literature. See, for example, Behzad Razavi,RF Microelectronics, Prentice Hall (1998) and John G. Proakis, DigitalCommunications, McGraw-Hill (1995). There exists also patent art relatedto the technology of I/Q modulation and demodulation: U.S. Pat. No.5,974,306, entitled “Time-Share I/Q Mixer System With DistributionSwitch Feeding In-Phase and Quadrature Polarity Inverters” to Hornak, etal.; U.S. Pat. No. 5,469,126, entitled “I/Q Modulator and I/QDemodulator” to Murtojarvi.

Examples of system applications that incorporate and standardize I/Qmodulation and demodulation include the GSM (Global System for MobileCommunications), IS-136 (TDMA), IS-95 (CDMA), and IEEE 802.11 (wirelessLAN). I/Q modulation and demodulation have also been proposed for use inthe Bluetooth wireless communication systems.

Bluetooth is a low-power radio technology being developed with a view tosubstituting a radio link for wire and cable that now connect electronicdevices, such as personal computers, printers and a wide variety ofhandheld devices, including palm-top computers, and mobile telephones.The development of Bluetooth began in early 1998 and has been promotedby a number of telecommunications and computer industry leaders. TheBluetooth specification is intended to be open and royalty-free and isavailable to potential participants as a guide to the development ofcompatible products.

The Bluetooth system operates in the 2.4 GHz ISM (Industrial,Scientific, Medical) band, and devices equipped with Bluetoothtechnology are expected to be capable of exchanging data at speeds up to720 Kbs at ranges up to 10 meters. This performance is achieved using atransmission power of 1 mw and the incorporation of frequency hopping toavoid interference. In the event that a Bluetooth-compatible receivingdevice detects a transmitting device within 10 meters, the receivingdevice will automatically modify its transmitting power to accommodatethe range. The receiving device is also required to operate in alow-power mode as traffic volume becomes low, or ceases altogether.

Bluetooth devices are capable of interlinking to form piconets, each ofwhich may have up to 256 units, with one master and seven slaves activewhile others idle in a standby mode. Piconets can overlap, and slavescan be shared. In addition, a form of scatternet may be established withoverlapping piconets, thereby allowing data to migrate across thenetworks.

An example of a Bluetooth-compliant digital communications receiver thatincorporates an I/Q demodulator is depicted in FIG. 1. As may be seenfrom FIG. 1, the receiver includes an antenna 10 that intercepts atransmitted RF signal. The signal received by antenna 10 is filtered ina RF bandpass filter (BPF) 11. BPF 11 may be fixed-tuned or tunable andwill have a nominal center frequency at the anticipated RF carrierfrequency. The bandwidth of BPF will be designed as appropriate to theoverall receiver system design requirements and constraints. One salientpurpose of BPF 11 is to effect rejection of out-of-band RF signals, thatis, rejection of signals at frequencies other than the frequency of thedesired RF carrier. Front-end selectivity is an important factor inminimizing the receiver's susceptibility to intermodulation andcross-modulation interference. In addition, and contextually morerelevant, BPF 11 selectivity contributes to the image-rejectioncharacteristics of the receiver.

In general, image rejection refers to the ability of the receiver toreject responses resulting from RF signals at a frequency offset fromthe desired RF carrier frequency by an amount equal to twice theintermediate frequency (IF) of a dual-conversion receiver. For example,if the desired RF signal is at 100 MHz, and the receiver IF is 10.7 MHz,than the receiver local oscillator (LO) will be tuned to 89.3 MHz.However, as is well known to those skilled in the art, the receiver willalso exhibit a response to undesired RF signals at frequency 10.7 MHzbelow the LO frequency, that is 78.6 MHz. The receiver's response to the78.6 MHz signal is referred to as the image response, because the imagesignal resides at a frequency opposite the LO frequency as the desiredRF carrier, and offset from the LO frequency by the magnitude of the IF.

Referring still to FIG. 1, the output of BPF 11 is coupled to the inputof a low-noise amplifier (LNA) 12. LNA 12 is designed to raise the levelof the input RF signal sufficiently to effectively drive the receiver'smixer circuitry. In addition, LNA 12 largely determines the receiver'snoise figure.

The output of LNA 12 is coupled to the receiver's mixer/demodulatorfunctional block. The mixer/demodulator includes a quadraturedemodulator, including I demodulator 13 and Q demodulator 14. As iscommonplace in contemporary receiver design, the receiver incorporates adigital, frequency-synthesized LO function, performed by avoltage-controlled oscillator (VCO) 15, driven by a phase-locked loop(PLL) 16. For a comprehensive exposition of digital frequency-synthesistechniques, see William F. Egan, Frequency Synthesis by Phase Lock, JohnWiley & Sons, Inc., (2000). The LO signal is coupled to an input ofphase-shifter 17. In a manner well understood by artisans, phase-shifter17 delivers an in-phase version of the LO, LO_I signal 13 a, to Idemodulator 13 and a quadrature (90° phase shifted) version of the LO,LO_Q signal 14 a, to Q demodulator 14. The respective demodulatedoutputs of demodulators 13 and 14 constitute, respectively, thedemodulated I and Q signals.

An ideal I/Q demodulation receiver, as described above, is theoreticallycapable of infinite image rejection. However, the theoretical assumptionis predicated on perfectly matched I and Q channels. Becausestate-of-the art semiconductor device design and fabrication does notadmit of perfect matching between devices, even devices on the same die,some degree of mismatch between the I and Q channels is inevitable. Infact, the mismatch between devices on a semiconductor wafer is known tobe dependent on the physical size of the devices. This dependency may bepredicted by the following relationships that quantify the standarddeviation in threshold voltage σ_(Vt), and β,σ_(β), for a MOS device:$\sigma_{Vt} = {\frac{30\quad \left( {\text{millivolt} - \text{micrometer}} \right)}{\sqrt{W \times L}}\quad \text{and}}$${\sigma_{\beta} = \frac{0.09\quad \left( \text{micrometer} \right)}{\sqrt{W \times L}}},\quad \text{where}$

W×L is total area occupied by the device on the semiconductor die.

As is immediately apparent from the above, deviations in critical CMOSdevice parameters vary inversely with to the area occupied by thedevice. Because lower frequencies of operation permit larger devicegeometries, mismatch in a receiver IF section tends to be ameliorated asthe IF is reduced.

It has been empirically determined that contemporary semiconductorfabrication processes result in I channel and Q channel matching thatlimits image rejection to approximately 30 to 35 db. In systemsimplemented with CMOS technology, virtually mandatory when powerconsumption is a paramount design consideration, not even this level ofperformance is realizable. This detriment derives from the fact thatCMOS devices tend to demonstrate less favorable matchingcharacteristics. Given that a 35 db image rejection specification isconsidered marginal for most digital communication receiverapplications, the problems confronted in a CMOS-based design areglaringly apparent.

Accordingly, what is desired is a solution that enhances theimage-rejection performance of digital communication receivers that areimplemented with integrated circuit technology. Although the solution isnot limited in applicability of designs implemented in CMOS technology,the invention is particularly advantageous in that context.

SUMMARY OF THE INVENTION

The above and other objects, advantages and capabilities are achieved inone aspect of the invention by a communications receiver that comprisesa carrier signal source; a first demodulator having a first inputcoupled to an output of the carrier signal source; a second demodulatorhaving a first input coupled to an out put of the carrier signal source;a local oscillator (LO) signal source; a quadrature phase shifter havingan LO input coupled to the LO signal source, an in-phase (I) outputcoupled to a second input of the first demodulator, and quadrature (Q)output coupled to a second input of the second demodulator; and animage/signal ratio detector having a first input coupled to an output ofthe first demodulator, a second input coupled to an output of the seconddemodulator, and an output coupled to the quadrature phase shifter foradjusting the I output of phase shifter and the Q output of phaseshifter so as to adjust the image response of the communicationsreceiver.

Another aspect of the invention is apparent in a feedback loop forcontrolling the image response of a communications receiver that,typically, includes a carrier signal source, a local oscillator (LO)signal source, an in-phase (I) demodulator and a quadrature (Q)demodulator. The feedback loop comprises a quadrature LO generator forproviding an LO_I signal to the I demodulator and an LO_Q signal to theQ demodulator, wherein the LO_I and LO_Q signals areamplitude-controlled and phase-controlled versions of the LO signalprovided by the LO signal source. The feedback loop also comprises animage/signal ratio (I/S) detector for detecting the amplitude differenceand the phase difference between the respective outputs of the Idemodulator and the Q demodulator and for adjusting the respectiverelative amplitudes and phases of the LO_I and LO_Q signals in responseto the detected amplitude difference and phase difference.

The invention may also be practiced as a method for adjusting the imageresponse of a communications receiver that includes in-phase (I) andquadrature (Q) demodulators. The method comprises the acts: synthesizingan LO signal; deriving an LO_I signal from the LO signal; deriving anLO_Q signal from the LO signal; detecting an amplitude control signalthat results from an amplitude mismatch between an I channel and a Qchannel of the communications receiver; detecting a phase control signalthat results from a phase mismatch between the I channel and the Qchannel; adjusting the relative amplitudes of the LO_I and the LO_Qsignals in response to the amplitude control signal; and adjusting therelative phases of the LO_I and the LO_Q signals in response to thephase control signal, wherein the adjustments to the relative respectiveamplitudes and the relative respective phases of the LO_I and LO₁₃ Qsignals operate to compensate for mismatch between the I channel and theQ channel in a manner that reduces the image response of the receiver.

The invention is additionally embodied in a mixer for a communicationsreceiver. The mixer comprises an I demodulation channel including an Idemodulator; a Q demodulation channel including a Q demodulator; aquadrature LO generator for coupling to a source of LO signals, thequadrature LO generator for developing an LO₁₃ I signal coupled to the Idemodulator and an LO_Q signal coupled to the Q demodulator, wherein theLO_I and LO₁₃ Q signals are adjusted in amplitude and phase in responseto mismatch between the I and Q channels, wherein the quadrature LOgenerator comprises: a polyphase filter having an input for coupling tothe source of LO signals, and having an LO_I output and an LO_Q output;an LO_I buffer having an LO_I input coupled to the LO_I output of thepolyphase filter and having an LO_I output coupled to the I demodulator;and an LO_Q buffer having an LO_Q input coupled to the LO_Q output ofthe polyphase filter and having an LO_Q output coupled to the Qdemodulator; and an I/S detector for synthesizing an amplitude controlsignal and a phase control signal in response to mismatch between the Iand the Q channels, the I/S detector having (i) inputs coupled tooutputs of the I and the Q demodulators, (ii) an output for applying anamplitude control signal to an amplitude control input of the quadratureLO generator, and (iii) an output for applying an phase control signalto a phase control input of the quadrature LO generator, the I/Sdetector comprising: a rotator having inputs coupled to the outputs ofthe I and the Q demodulators and having first and second I and Qoutputs; an amplitude meter coupled to the first I and Q output of therotator for developing the amplitude control signal; and a phase metercoupled to the second I and Q output of the rotator for developing thephase control signal.

The invention may also be perceived as a method for calibrating acommunications receiver that includes (i) an I demodulation channelincluding an I demodulator, (ii) a Q demodulator channel including a Qdemodulator, and (iii) a quadrature LO generator having an input coupledto a source of LO signals and that provides an LO_I signal to the Idemodulator and an LO_Q signal to the Q demodulator. The calibrationmethod comprises the acts: (a) during an interval during which noinformation is received by the communications receiver, applying an RFtest tone to the inputs of the I demodulation channel and the Qdemodulation channel; (b) time multiplexing a first LO signal and asecond LO signal to the input of the quadrature LO generator so as tosimulate the appearance of both a desired RF signal and an image signalat the input of the communications receiver; (c) detecting a signal,V_(S,IM), resulting from the response of the communications receiver tothe simulated image signal; (d) extracting from V_(S,IM) an amplitudecontrol signal is proportional to the amplitude mismatch between the Iand the Q channels and a phase control signal that is proportional tothe phase mismatch between the I and the Q channels; and (e) adjustingthe relative amplitudes of the LO_I and the LO_Q signals in response tothe amplitude control signal and adjusting the relative phases of theLO_I and the LO_Q signals in response to the phase control signalwherein the adjustments to the relative respective amplitudes and therelative respective phases of the LO_I and LO_Q signals operate toreduce the image response of the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject invention may be better understood, and its numerousobjects, features and advantages made apparent to those skilled in theart, with reference to the following Drawings, wherein:

FIG. 1 is a system block diagram of a conventional digitalcommunications receiver predicated on I/Q demodulation.

FIG. 2 is a system block diagram of an I/Q demodulation receiver thatincorporates aspects of the subject invention, including a quadrature LOgenerator, with amplitude and phase control provided by an I/S detector.

FIG. 3 is a block diagram of an Image/Signal Ration (I/S) Detector usedin connection with the subject invention.

FIG. 4A is a block diagram of the quadrature LO generator depicted inFIG. 2, including a polyphase filter and gain-controlled LO_I and LO₁₃ Qbuffers.

FIG. 4B is a generalized circuit diagram of the polyphase filter.

FIG. 4C is a simplified circuit diagram illustrating an example of themanner in which the polyphase filter may be tuned (phase-controlled)with a voltage-variable capacitor.

FIG. 4D is a circuit diagram of the buffers depicted in FIG. 4A.

DETAILED DESCRIPTION

For a thorough understanding of the subject invention, reference is madeto the following Description, including the appended Claims, inconjunction with the above described Drawings.

Referring now to FIG. 2, the subject receiver with image-rejectiondigital I/Q demodulator is seen to be in many respects similar to thedigital I/Q demodulation receiver depicted in FIG. 1. A salientdeparture is the substitution of the phase shifter 17 in FIG. 1 with thequadrature LO generator 21 in the receiver of FIG. 2. In addition, andin a manner to be more completely described below, the operation ofquadrature LO generator 21 is controlled by an image/signal ratio (I/S)detector 22 that applies amplitude control and phase control signals toquadrature LO generator 21 in a manner that adjusts the LO_I and LO_Qsignals to the I demodulator 13 and to the Q demodulator 14respectively, so as to reduce the image response of the receiver.

With continued reference to FIG. 2, operation of subject invention maybe understood to proceed as follows. The demodulated I output from Idemodulator 13 is fed into one input of I/S detector 22, and thedemodulated Q output of Q demodulator 14 is fed into a second input ofthe I/S detector. A detailed depiction of the I/S detector is providedin FIG. 3. I/S detector 22 is seen in FIG. 3 to include a pair of A/Dconverters 211 and 212, respectively, at the input of the I/S detector.In practice, A/D converters 211 and 212 may both be 10-bit converters.However, A/D conversion at the input of the I/S detector is requiredonly in systems that deliver analog outputs at demodulators 13 and 14.In systems that provide digital signals at the demodulator outputs, A/Dconversion at the I/S detector input may be eliminated. The outputs ofthe A/D converters are coupled to the respective I and Q inputs of 90°rotator 213. Rotator 213 imparts an additional 90° phase shift betweenthe I and the Q signals, so that at the output of the rotator, the I andQ signals are 180° out of phase. Rotator 213 provides a first pair of Iand Q signals from terminals 213 a and 213 b to the inputs of amplitudemeter 214 and provides a second pair of I and Q output signals fromterminals 213 c and 213 d to the inputs of phase meter 215. Theamplitude control output 22 c of the I/S detector is applied to anamplitude control input of the quadrature LO generator 21. The phasecontrol output 22 d of the I/S detector is applied to the phase controlinput of quadrature LO generator. As may be anticipated, the amplitudecontrol signal at output 22 c is proportional to the relative amplitudesof the demodulated I and Q signals. Similarly, the phase control signalat output 22 d is proportional to the phase difference between thedemodulated I and Q signals. The amplitude control signal and the phasecontrol signal are measures of the mismatch between the I and Qchannels. The manner in which the control signals are synthesized may beacquired with continued reference to FIG. 3.

Simply, operation of I/S detector 22 is predicated on the application ofa test tone, at the anticipated RF carrier frequency, to the input ofthe mixer of the digital receiver, that is, at the node occupied incommon by the inputs of the I and Q demodulators. The LO frequency isadjusted first to run at a frequency (RF−IF), and then at a frequency(RF+IF). Consequently, IF signals are generated in a manner that isgenerally equivalent to the appearance of input signals at both theanticipated RF carrier frequency, and at an image frequency. Viewedalternatively, the variation in LO frequency simulates the appearance ofan image signal at the input of the receiver.

The two IF signals, due to desired RF carrier and to image signal,respectively, are time duplexed and appropriately processed into a firstsignal that is proportional to the amplitude difference between LO_I andLO_Q, and into a second signal that is proportional to the phasedifference between LO_I and LO_Q. The theoretical basis for this signalprocessing follows.

Assume that the RF and image signals are respectively referred to asV_(RF) and V_(IM). Then the down-converted IF signals are V_(I,RF),V_(Q,RF), V_(J,IM), and V_(Q,IM), respectively. After down conversion,and filtering of high-frequency components, the following signalsremain:

V _(LO) =V ₂ cos(ω_(LO) t)

V _(I,RF)=½V ₁ V ₂ cos(ω_(IF) t)

V _(Q,RF)=−½V ₁ V ₂ cos(ω_(IF) t)

V _(I,IM)=½V ₁ V ₂ cos(ω_(IF) t)

V _(Q,IM)=½V ₁ V ₂ cos(ω_(IF) t)

Ideally, subtraction of the I and Q components of the RF and IM signalsresults in the relationships:

V _(IF,RF) =V ₁ V ₂ cos(ω_(IF) t)

V _(IF,IM)=0.

The results above indicate that if the I and the Q demodulator channelsare precisely matched, V_(IF,IM)=0. That is, the image signal will beentirely rejected.

However, as suggested above, realizable semiconductor implementations,specifically, CMOS implementations, do not enable precisely matched Iand Q channels. Mismatch between the I and Q channels may be modeledmathematically as mismatch between the LO_I and the LO_Q signals 13 band 14 b, respectively. Accordingly:

V _(LO,I) =V ₂ cos(ω_(LO) t)

V _(LO,Q)=(V ₂ +ΔV ₂)cos(ω_(LO) t+ΔΦ),

where ΔV and ΔΦ represent the amplitude mismatch and the phase mismatch,respectively, of the I and Q channels. As a result an image componentwill contaminate the down-converted IF signal. The summed signalcorresponding to the image response is:

V _(S,IM)=−½V ₁ cos(ω_(IF) t)ΔV+{fraction (1/2)}V ₁ V ₂ sin(ω_(IF) t)ΔΦ.

It is apparent that unwanted image signal consists of two components.The first component is proportional to the amplitude mismatch betweenthe I and Q channels; and the second component is proportional to thephase mismatch. A straightforward mathematical operation, as may beimplemented in one of many commercially available DSP integrated circuitdevices, may be employed to extract the amplitude error signal and thephase error signal. To wit: V_(S,IM) may be multiplied by cos(ω_(IF)t)to extract the amplitude error signal, and by sin (ω_(IF)t) to extractthe phase error signal.

In operation of the receiver, the amplitude control signal and the phasecontrol signal are synthesized at intervals during which the receiver isnot engaged in the reception and processing of information. For example,a Bluetooth receiver, in accordance with the subject invention,calibrates the I and Q channels during the interstitial time periodsbetween reception of data packets. During the dormant interstitialperiods, an RF test tone is applied to the input of the mixer section ofthe receiver, that is, to the inputs of both the I demodulator and the Qdemodulator. Concurrently, disparate LO signals are applied in atime-multiplexed mode, to the input of quadrature LO generator. One ofthe LO signals runs at the frequency appropriate to the RF test tone soas to result in a desired IF output from the demodulators. The second LOsignal is frequency offset from the appearance at the receiver input, animage signal. The response of the I and Q channels to the simulatedimage is then detected as described above. The amplitude control signaland the phase control signal are used to calibrate the I and Q channelsagainst mismatch.

In addition, numerous approaches are available to undertake the signalprocessing inherent in the subject invention. It is understood that therequisite signal processing may be achieved in hardware, software or acombination of the two. The partitioning of these functions is largelywith discretion of the receiver designer. However, inasmuch as thesubject invention is intended to be realized, so far as practicable, inmonolithic integrated circuit technology, it follows that use of one ofthe many commercially available digital signal processing (DSP) devicesis advantageous. DSP devices and techniques are well known to skilledartisans. See, for example, Ralph Chassaing, Digital Signal Processing:Laboratory Experiments Using C and the TMS320C31 DSK (Wiley-Intersorence1999). If the calibration signals, at both the RF and image, aredesigned to span the entire dynamic range of the A/D converter, then theachievable image rejection is equivalent to the dynamic range of the A/Dconverter. For a 10-bit A/D converter, 60 db image rejection istherefore obtainable.

To this point, a technique for developing both an amplitude controlsignal, proportional to ΔV, and a phase control signal, proportional toΔΦ, has been fully explicated. Reiterating, the amplitude control signalappears at output 22 c, and the phase control signal at output 22 d, ofI/S ratio detector 22. As may be seen from FIG. 2, these control signalsare applied, respectively, to inputs 21 a and 21 b of quadrature LOgenerator 21, in order to control the amplitude and phase differencesbetween LO_I, appearing at output 21 d, and LO_Q, appearing at output 21e, of the quadrature LO generator. A preferred implementation of thequadrature LO generator is depicted in FIGS. 4A, 4B and 4C.

Directing attention first to FIG. 4A, the quadrature LO generator isseen to include a polyphase filter 41 having a balanced (differential)input 41 a coupled to the output of the phase-locked local oscillator.The differential input to the polyphase filter consists of an LOP and anLON input. The polyphase filter has a first differential output 41 ccoupled to LO_I buffer 42 and a second differential output 41 d coupledto LO₁₃ Q buffer 43. The phase control output of I/S detector 22 iscoupled to phase control input 41 b of the polyphase filter. Theamplitude control output of I/S detector 22 is coupled to amplitudecontrol input 42 b of LO_I buffer 42 and to the amplitude control input43 b of LO_Q buffer 43.

An exemplary embodiment of the polyphase filter is depicted in FIG. 4B,in which the polyphase filter may be seen to be implemented in the formof an array of tunable capacitors. The array illustrated in FIG. 4B maybe defined in terms the set of circuit nodes that consist of input nodesLOP and LON, output nodes LO_IP, LO_QP, LO_IN and LO_QN, internal nodes421, 422, 423, 424, and a reference node GND. Each of the internal nodesis coupled through an associated capacitance to a respective outputnode. In addition, node 421 is coupled through a capacitance to nodeGND, node 422 is coupled through a capacitance to node LOP, node 423 iscoupled through a capacitance to node GND, and node 424 is coupledthrough a capacitance to node LON. Also, each of the internal nodes,421, 422, 423 and 424, is coupled through an associated resistance to anoutput node. In addition, node 421 is coupled through a resistance tonode LOP, node 422 is coupled through a resistance to node GND, node 423is coupled through a resistance to node LON, an node 424 is coupledthrough a resistance to node GND. The values for the resistances andcapacitances depicted in the polyphase filter array of FIG. 4B aredetermined primarily with respect to the operating frequency of the LOsignal source and are easily determined by those skilled in the art.FIG. 4C depicts the manner in which a phase control voltage may beapplied to a voltage-tunable capacitance, such as a varactor diode, in amanner that will vary the capacitance of the diode.

The output nodes of the polyphase filter are coupled to an LO_I bufferand to an LO_Q buffer that effect amplitude control in response to theamplitude control signal supplied by the I/S detector. Specifically, thedifferential LO_IP and LO_IN output of the polyphase filter are appliedto the differential input of LO_I buffer 42, and the differential LO_QPand LO_QN output of the polyphase filter are coupled to the differentialinput of LO_Q buffer 43. For the purposes of this Description, the LO_Iand LO_Q buffers may be understood to be substantially identical inform. An exemplary embodiment of such a buffer is provided in FIG. 4D.

As seen in FIG. 4D, each buffer is constructed around a matched pair oftransistors, M1 and M2, arranged in a differential amplifierconfiguration. In each buffer, the respective differential LO_I or LO_Qinput is applied to the input of the differential amplifier, at gateelectrodes of M1 and M2. M1 and M2 are MOSFETS configured in asource-coupled mode. Amplitude control of the input LO_I and LO_Qsignals is effected by controlling the bias current flowing in M1 andM2. In the arrangement of FIG. 4D a bank of constant-current sources441, 442, 443 and 444 are coupled in parallel to the common sources ofM1 and M2. The respective magnitudes of the currents sourced by sources441, 442, 443 and 444 are binary weighted, so that the current of source442 is twice the current of source 441, the current of source 443 istwice the current of 442, and so forth. As may be expected, the binaryamplitude control signal from the amplitude meter of I/S detector 42operates to selectively render the separate binary-weighted currentsources conductive or non-conductive, thereby varying the gain impartedby the differential amplifier to the input LO_I or LO_Q signal. Assuggested in FIG. 4D, it is deemed preferable that at least one of thecurrent sources remain continuously conductive so that M1 and M2 arealways biased with a nominal quiescent current flow. To this end,current source 444 remains continuously conductive.

Although the subject invention has been described in detail in thecontext of the exemplary embodiments presented above, the invention, isnot to be limited to the described embodiments, but is to be afforded ascope commensurate with the appended Claims, and substantial equivalentsthereof. Those having ordinary skill in the art may readily comprehendvarious additions, modifications and improvements to the describedembodiments of the invention, and all such modifications are to bedeemed within the scope of the invention.

What is claimed is:
 1. A communications receiver comprising: a carriersignal source; a first demodulator having a first input terminal coupledto an output terminal of the carrier signal source; a second demodulatorhaving a first input terminal coupled to an output terminal of thecarrier signal source; a local oscillator (LO) signal source; aquadrature phase shifter having an LO input terminal coupled to the LOsignal source, an in-phase (I) output terminal coupled to a second inputterminal of the first demodulator, and a quadrature (Q) output terminalcoupled to a second input terminal of the second demodulator; and animage/signal ratio detector having a first input terminal coupled to anoutput terminal of the first demodulator, a second input terminalcoupled to an output terminal of the second demodulator, and an outputterminal coupled to the quadrature phase shifter for adjusting the Ioutput of the phase shifter and the Q output of the phase shifterthereby to adjust the image response of the communications receiver. 2.A communications receiver as defined in claim 1, wherein theimage/signal ratio detector is operable to adjust the phase and theamplitude of the I output of the phase shifter and to adjust the phaseand the amplitude of the Q output of the phase shifter thereby to reducethe image response of the communications receiver.
 3. In acommunications receiver having a carrier signal source, a localoscillator (LO) signal source, an in-phase (I) demodulator and aquadrature (Q) demodulator, a feedback loop for controlling the imageresponse of the communications receiver, the feedback loop comprising: aquadrature LO generator coupled to provide an LO_I signal (13 a) to theI demodulator and an LO_Q signal (14 b) to the Q demodulator, whereinthe LO_I and LO_Q signals are amplitude-controlled and phase-controlledversions of the LO signal provided by the LO signal source; and animage/signal ratio (I/S) detector coupled to detect the amplitudedifference and the phase difference between the respective outputs ofthe I demodulator and the Q demodulator and to adjust the respectiverelative amplitudes and phases of the LO_I and LO_Q signals in responseto the detected amplitude difference and phase difference.
 4. A feedbackloop for controlling the image response of a communications receiver asdefined in claim 3, wherein the quadrature LO generator has an LO inputterminal for coupling to the LO signal source, an LO_I output terminalfor coupling the LO_I signal to the I demodulator, an LO_Q outputterminal coupling the LO₁₃ Q signal to the Q demodulator, an amplitudecontrol input terminal and a phase control input terminal and whereinthe I/S detector has an I input terminal coupled to the output terminalof the I demodulator, a Q input terminal coupled to the output terminalof the Q demodulator, an amplitude control output terminal coupled tothe amplitude control input terminal of the quadrature LO generator foradjusting the relative amplitudes of the LO₁₃ I signal and the LO_Qsignal, and a phase control output terminal coupled to the phase controlinput terminal of the quadrature LO generator for adjusting the relativephases of the LO_I signal and the LO_Q signal.
 5. A feedback loop forcontrolling the image response of a communications receiver as definedin claim 4, wherein the relative amplitudes of the LO_I signal and theLO_Q signal and the relative phases of the LO_I signal and the LO₁₃ Qsignal are adjusted thereby to reduce the image response of thecommunications receiver.
 6. A feedback loop for controlling the imageresponse of a communications receiver as defined in claim 4, wherein thequadrature LO generator comprises: a polyphase filter having an LO inputterminal coupled to the LO signal source and a phase control inputterminal coupled to the phase control output terminal of the I/Sdetector; an LO_I buffer having an LO₁₃ I input terminal coupled to anLO_I output terminal of the polyphase filter and an amplitude controlinput terminal coupled to the amplitude control output terminal of theI/S detector; and an LO_Q buffer having an LO_Q input terminal coupledto an LO₁₃ Q output terminal of the polyphase filter and an amplitudecontrol input terminal coupled to the amplitude control output terminalof the I/S detector.
 7. A feedback loop for a controlling the imageresponse of a communications receiver as defined in claim 6, wherein thepolyphase filter comprises an array of capacitors that are tunable inresponse to the phase control output of the I/S detector.
 8. A feedbackloop for controlling the image response of a communications receiver asdefined in claim 7, wherein the LO_I buffer and the LO_Q buffer eachcomprises a differential amplifier having an input terminal coupled toan output terminal of the polyphase filter, and the gain of each of thedifferential amplifiers is controlled by varying the bias current in therespective differential amplifier in response to the gain control outputof the I/S detector.
 9. A feedback loop for controlling the imageresponse of a communications receiver as defined in claim 8, wherein therelative amplitude of the LO_I signal and the LO_Q signal and therelative phases of the LO_I signal and the LO_Q signal are adjustedthereby to reduce the image response of the communications receiver. 10.A feedback loop for controlling the image response of a communicationsreceiver as defined in claim 6, wherein the LO_I buffer and the LO_Qbuffer each comprises a differential amplifier having an input terminalcoupled to an output terminal of the polyphase filter and the gain ofeach of the differential amplifiers is controlled by varying the biascurrent in the respective differential amplifier in response to the gaincontrol output of the I/S detector.
 11. A feedback loop for controllingthe image response of a communications receiver as defined in claim 10,wherein the relative amplitudes of the LO_I signal and the LO_Q signaland the relative phases of the LO_I signal and the LO_Q signal areadjusted thereby to reduce the image response of the communicationsreceiver.
 12. A feedback loop for controlling the image response of acommunications receiver as defined in claim 4, wherein the I/S detectorcomprises: a rotator having an I input terminal coupled to the outputterminal of the I demodulator and a Q input terminal coupled to theoutput terminal of the Q demodulator, a first output terminal, and asecond output terminal, the rotator thereby adjusting the relative phasebetween the output of the I demodulator and the output of the Qdemodulator so as to establish a relative phase substantially equal to180°; an amplitude meter having an input terminal coupled to the firstoutput terminal of the rotator and providing an amplitude control signalat an output terminal; and a phase meter having an input terminalcoupled to the second output terminal of the rotator and providing aphase control signal at an output terminal.
 13. A feedback loop forcontrolling the image response of a communication receiver as defined inclaim 12, wherein the I input terminal and the Q input terminal of therotator means are each coupled to the respective output terminal of theI demodulator and the output terminal of the Q demodulator through acorresponding A/D converter.
 14. A feedback loop for controlling theimage response of a communications receiver as defined in claim 12,wherein the quadrature LO generator comprises: a polyphase filter havingan LO input terminal coupled to the LO signal source and a phase controlinput terminal coupled to the phase control output terminal of the I/Sdetector; an LO_I buffer having an LO₁₃ I input terminal coupled to anLO_I output terminal of the polyphase filter and an amplitude controlinput terminal coupled to the amplitude control output terminal of theI/S detector; and an LO₁₃ Q buffer having an LO_Q input terminal coupledto an LO_Q output terminal of the polyphase filter and an amplitudecontrol input terminal coupled to the amplitude control output terminalof the I/S detector.
 15. A feedback loop for a controlling the imageresponse of a communications receiver as defined in claim 14, whereinthe polyphase filter comprises an array of capacitors that are tunablein response to the phase control output of the I/S detector.
 16. Afeedback loop for controlling the image response of a communicationsreceiver as defined in claim 15, wherein the LO_I buffer and the LO_Qbuffer each comprises a differential amplifier having an input terminalcoupled to an output terminal of the polyphase filter, and the gain ofeach of the differential amplifiers is controlled by varying the biascurrent in the respective differential amplifier in response to the gaincontrol output of the I/S detector.
 17. A feedback loop for controllingthe image response of a communications receiver as defined in claim 16,wherein the relative amplitudes of the LO_I signal and the LO_Q signaland the relative phases of the LO_I signal and the LO_Q signal areadjusted thereby to reduce the image response of the communicationsreceiver.
 18. A method for adjusting the image response of acommunications receiver that includes in-phase (I) and quadrature (Q)demodulators, the method comprising the acts: synthesizing an LO signal;deriving an LO_I signal from the LO signal; deriving an LO_Q signal fromthe LO signal; detecting an amplitude control signal that results froman amplitude mismatch between an I channel and a Q channel of thecommunications receiver; detecting a phase control signal that resultsfrom a phase mismatch between the I channel and the Q channel; adjustingthe relative amplitudes of the LO_I and the LO_Q signals in response tothe amplitude control signal; and adjusting the relative phases of theLO_I and the LO_Q signals in response to the phase control signal,wherein the adjustments to the relative respective amplitudes and therelative respective phases of the LO_I and LO_Q signals operate tocompensate for mismatch between the I channel and the Q channel therebyto reduce the image response of the receiver.
 19. A method for adjustingthe image response of a communications receiver as defined in claim 18,wherein the amplitude control signal is detected by multiplying an imagesignal, V_(S,IM), by cos (ω_(IF)t) and the phase control signal isdetected by multiplying V_(S,IM) by sin (ω_(IF)t), where ω_(IF) is theangular intermediate frequency (IF) of the receiver.
 20. A method ofadjusting the image response of a communications receiver as defined inclaim 19, wherein the LO_I and LO_Q signals are derived from the LOsignal by a quadrature LO generator, and the amplitude control signaland the phase control signal are detected by an I/S detector.
 21. Amethod for adjusting the image response of a communications receiver asdefined in claim 20, wherein the quadrature LO generator comprises: apolyphase filter having a first input terminal for coupling to the LOsignal, a phase control input terminal coupled to the I/S detector, andLO_I output terminal, and an LO_Q output terminal; a LO_I buffer havingan LO_I input terminal coupled to the LO_I output terminal of thepolyphase filter and an amplitude control input terminal coupled to theamplitude control output terminal of the I/S detector; and a LO_Q bufferhaving an LO_Q input terminal coupled to the LO₁₃ Q output terminal ofthe polyphase filter and an amplitude control input terminal coupled tothe amplitude control output terminal of the I/S detector.
 22. A methodfor adjusting the image response of a communications receiver as definedin claim 18, wherein the amplitude control signal and the phase controlsignal are detected in an I/S detector that comprises: a rotator havinginput terminals coupled to the respective output terminals of the I andthe Q demodulators and having first (213 a, 213 b) and second (213 c,213 d) (I,Q) output terminals; an amplitude meter coupled to the first(I,Q) output terminal of the rotator for detecting the amplitude controlsignal; and a phase meter coupled to the second (I,Q) output terminal ofthe rotator for detecting the phase control signal.
 23. A method foradjusting the image response of a communications receiver as defined inclaim 22, wherein the quadrature LO generator comprises: a polyphasefilter having an input terminal for coupling to the source of LO signalsand having an LO_I output terminal and an LO_Q output terminal; an LO_Ibuffer having an LO₁₃ I input terminal coupled to the LO_I outputterminal of the polyphase filter and having an LO_I output terminalcoupled to the demodulator; and an LO_Q buffer having an LO_Q outputterminal of the polyphase filter and having an LO_Q output terminalcoupled to the Q demodulator.
 24. A mixer for a communications receiver,the mixer comprising: an I demodulation channel including an Idemodulator; a Q demodulation channel including a Q demodulator; aquadrature LO generator for coupling to a source of LO signals, thequadrature LO generator for developing an LO_I signal coupled to the Idemodulator and an LO₁₃ Q signal coupled to the Q demodulator, whereinthe LO_I and LO₁₃ Q signals are adjusted in amplitude and phase inresponse to mismatch between the I and Q channels, wherein thequadrature LO generator comprises: a polyphase filter having an inputterminal for coupling to the source of LO signals, and having an LO_Ioutput terminal and an LO_Q output terminal; an LO_I buffer having anLO_I input terminal coupled to the LO_I output terminal of the polyphasefilter and having an LO_I output terminal coupled to the I demodulator;and an LO_Q buffer having an LO_Q input terminal coupled to the LO_Qoutput terminal of the polyphase filter and having an LO_Q outputterminal coupled to the Q demodulator; and an I/S detector forsynthesizing an amplitude control signal and a phase control signal inresponse to mismatch between the I and the Q channels, the I/S detectorhaving (i) input terminals coupled to output terminals of the I and theQ demodulators, (ii) a first output terminal for applying an amplitudecontrol signal to an amplitude control input terminal of the quadratureLO generator, and (iii) a second output terminal for applying an phasecontrol signal to a phase control input terminal of the quadrature LOgenerator, the I/S detector comprising: a rotator having input terminalscoupled to the output terminals of the I and the Q demodulators andhaving first and second I and Q output terminals; an amplitude metercoupled to the first I and Q output terminal of the rotator fordeveloping the amplitude control signal; and a phase meter coupled tothe second I and Q output terminal of the rotator for developing thephase control signal.
 25. A mixer for a communications receiver asdefined in claim 24, wherein the LO_I and LO_Q signals are adjusted in amanner to reduce the image response of the receiver.
 26. A mixer for acommunications receiver as defined in claim 25, wherein the I/S detectordetects a signal, V_(S,IM), that is representative of the image responseof the communications receiver and extracts from V_(S,IM) the amplitudecontrol signal and the phase control signal.
 27. A mixer for acommunications receiver as defined in claim 26, wherein the amplitudecontrol signal is extracted from V_(S,IM) by multiplying V_(S,IM) by cos(ω_(IF)t) and the phase control signal is extracted from V_(S,IM) bymultiplying V_(S,IM) by sin (ω_(IF)t), where ω_(IF) is the angularintermediate frequency of the communications receiver.
 28. A mixer for acommunications receiver as defined in claim 27, wherein the polyphasefilter comprises an array of tunable capacitances.
 29. A mixer for acommunications receiver as defined in claim 28, wherein the array oftunable capacitances comprises capacitances that are voltage-tunable inresponse to the phase control signal.
 30. A mixer for a communicationsreceiver as defined in claim 29, wherein the LO_I buffer and the LO_Qbuffer each comprises a differential amplifier that has an inputterminal coupled to an output terminal of the polyphase filter and thatis gain-controllable in response to a digital amplitude control signal.31. A mixer for a communications receiver as defined in claim 30,wherein the differential amplifier comprises a pair of source-coupledMOSFETs.
 32. A mixer for a communications receiver as defined in claim30, wherein the LO_I buffer and the LO_Q buffer each comprises adifferential amplifier that has an input terminal coupled to an outputterminal of the polyphase filter and that is gain-controllable inresponse to a digital amplitude control signal.
 33. In a communicationsreceiver that includes (i) an I demodulation channel including an Idemodulator, (ii) a Q demodulator channel including a Q demodulator, and(iii) a quadrature LO generator having an input terminal coupled to asource of LO signals and that provides an LO_I signal to the Idemodulator and an LO_Q signal to the Q demodulator, a method ofcalibrating the receiver to compensate for mismatch between the I andthe Q channels, the method comprising the acts: (a) during an intervalduring which information is not received by the communications receiver,applying an RF test tone to the input terminals of the I demodulationchannel and the Q demodulation channel; (b) time multiplexing a first LOsignal and a second LO signal to the input terminal of the quadrature LOgenerator so as to simulate the appearance of both a desired RF signaland an image signal at the input terminal of the communicationsreceiver; (c) detecting a signal, V_(S,IM), resulting from the responseof the communications receiver to the simulated image signal; (d)extracting from V_(S,IM) an amplitude control signal that isproportional to the amplitude mismatch between the I and the Q channelsand a phase control signal that is proportional to the phase mismatchbetween the I and the Q channels; and (e) adjusting the relativeamplitudes of the LO_I and the LO₁₃ Q signals in response to theamplitude control signal and adjusting the relative phases of the LO_Iand the LO_Q signals in response to the phase control signal, whereinthe adjustments to the relative respective amplitudes and the relativerespective phases of the LO_I and LO_Q signals operate to reduce theimage response of the receiver.
 34. A method of calibrating a receiveras defined in claim 33, wherein the method is performed during timeperiods between reception of data packets.
 35. A method as defined inclaim 34, wherein the quadrature LO generator comprises: a polyphasefilter having an input terminal coupled to a source of LO signals, andhaving an LO_I output terminal and an LO_Q output terminal; an LO_Ibuffer having an LO_I input terminal coupled to the LO_I output terminalof the polyphase filter and having an LO_I output terminal coupled tothe I demodulator; and an LO_Q buffer having an LO_Q input terminalcoupled to the LO_Q output terminal of the polyphase filter and havingan LO_Q output terminal coupled to the Q demodulator.
 36. A method asdefined in claim 35, wherein the method is performed, in part, by an I/Sdetector that synthesizes the amplitude control signal and the phasecontrol signal, the I/S detector comprising: a rotator having inputterminals coupled to the output terminals of the I and the Qdemodulators and having first and second I and Q output terminals; anamplitude meter coupled to the first I and Q output terminal of therotator for developing the amplitude control signal; and a phase metercoupled to the second I and Q output terminal of the rotator fordeveloping the phase control signal.